Burning fossil fuels results in emission of gaseous CO2 and of SO2 in small quantities. The injection of limestone into the combustion chamber reduces the emission of SO2 in situ, but this reduction in the emission of SO2 is accompanied by production of secondary CO2 in addition to the CO2 produced by combustion of the carbon-containing materials. Because CO2 is a greenhouse gas, its emission must be limited, imposing total or partial capture of the CO2.
The CO2 contained in the combustion flue gases can be captured in various ways and stored downstream of the combustion installation. Firstly, it is captured by washing it with a solvent, such as monoethanolamine, which selectively solubilizes the CO2 contained in the flue gases, after which the solvent is regenerated by extracting the CO2 by a heating process involving injecting steam into a second reactor, after which the regenerated solvent is fed to the flue gas washing reactor. However, this solution requires treatment of the nitrogen acting as a necessary inert ballast for the combustion reaction and contained in the flue gases, of which CO2 accounts for typically around 15% by volume. This means that the CO2 capture installation must be rated in proportion to the quantity of nitrogen present. Furthermore, thermal regeneration has the drawback of necessitating large quantities of steam, which penalizes the energy efficiency of the electricity and/or steam production installation.
Replacing the nitrogen ballast contained in the combustion air with recycled CO2 and carrying out combustion using an O2/CO2 oxidizing agent was then envisaged. However, the oxygen has to be produced from air by an air separation unit and a cryogenic system that consumes a great deal of energy.
The above solutions are based on combustion and capture of CO2 at atmospheric pressure, but it is also possible to produce installations integrated into electricity production cycles for gasifying under pressure solid fuels, such as coal. Gasification under pressure produces a synthetic fuel gas containing CO, H2, CO2 and H2O. These gasification units are coupled to gas turbines and in this case the CO2 under pressure is recovered by washing it with the solvent under pressure, which is more favorable for subsequent transportation of the CO2, which must be effected in the supercritical liquid state at 150 bar. The gasification reaction uses oxygen under pressure produced by an air separator unit, which is costly. Apart from its complexity, the above type of installation is somewhat unreliable, as its availability is typically only 80%.
It is also known in the art to achieve combustion of gases (but not of solid materials) with integral CO2 recovery using a metal oxide as an oxygen vector. The oxide circulates between two reactors in which it is either oxidized in a circulating bed reactor by bringing it into contact with air or reduced by bringing it into contact with the gaseous fuel. This method has the advantage of not necessitating separation of air, since the oxide constitutes the oxygen vector, but it cannot be used with solid fuels. Also, these installations must be totally airtight between the two reactors so as not to pollute air depleted in O2 leaving the oxidation reactor with CO2 leaking from the reduction reactor. However, to use gaseous fuel, which is very costly, it may be more economical to use high-efficiency (60%) gas turbines associated with heat recovery installations for treating the flue gases, rather than boilers coupled to a lower efficiency (45%) steam turbine.
Finally, the CO2 can be captured using a calcium carbonate cycle where the carbonates are formed downstream of a circulating fluidized bed installation in a circulating fluidized bed contact reactor, decomposed in a second reactor by input of heat, and recycled to the reactor in which they are brought into contact with the flue gases containing CO2. The CO2 released in this way can be recovered for storage.
Unfortunately, all these post-combustion CO2 capture techniques have the drawback of increasing by a factor of up to two the investment cost of a conventional coal power station and require a large footprint.